I have virtually the same setup you describe. I have a debian testing
system and a netgear router at home. I regularly use putty 0.58 (and
earlier versions) to ssh into my home system from the office. I've never
encountered anything like you describe.

I see I'm using a DSA key, where you're apparently using RSA. Offhand I
doubt that's the problem, but you might try generating a different key.

You probably have some kind of port forwarding set up on the netgear
router. Maybe it's set up incorrectly, and the ssh client is being
connected to something else like an FTP server.

Also, I've read that to port-forward an FTP server, the firewall has to
watch the FTP command channel, open holes for each data connection, and
maybe even modify some packets. I don't know if this is accurate and I
don't know if little home office routers have this logic, but I see
you're using port 21, the FTP command port. Maybe your router is trying
to parse the ssh session as an FTP session and screwing things up.